I neck size only.
I have to say that I reload for a SMLE in 303British:
- these rifles have oversize chambers (for use in muddy trenches)
- .303Br head-spaces on the rim.
For the above mentioned reason NSO is preferable in my opinion and you get fire-formed brass for your reloads.
Do you gentlemen think I am on the right track?
"A 9mm might expand, but, a .45 will never shrink!"